The present invention relates to improvements in split-shell magnetic printing cylinders. Magnetic cylinders of the type with which the present invention is concerned are used in the printing industry to hold printing forms in place. The forms are provided on a curved ferrous plate or base which is held to the cylinder through magnetic attraction. The printing surface of the form is linked, and the image is transferred to a passing web or sheet in a conventional manner.
Magnetic printing cylinders have been known for some time. The cylinders have been formed by placing laminations on a machined core. The laminations are arranged so as to alternately form pole pieces and spacers. Two or more laminations of non-ferromagnetic material may form the spacers, and apertures are provided for holding permanent magnet pieces which engage alternate pole pieces.
For convenience of fabrication and assembly to a shaft, the cylinders are provided in complementary half-cylinders or "split-shells" as they are referred to in the industry. To facilitate fabrication of the laminations to the core, the pole pieces and spacers are formed from annular plates which are then split along a diameter so that each lamination has an outer curved edge and an inner curved edge. The outer curved edge conforms to the outer surface of the cylinder on which the printing form is placed, and the inner curved edge is shaped to conform to a core on which the laminations are placed.
In the past, a core has been formed by machining a recess for the laminations from steel stock having a much larger diameter than is necessary to receive the laminations, thereby leaving a first end plate for securing the laminations by means of tie rods. Thus, after machining, an integral core piece remains comprising an end plate and a central column or hub in the form of a half cylinder on which the laminations were placed. To complete the assembly, a second end plate was welded to the distal end of the hub and the end plates and laminations were secured together by tie rods extending through the end plates and laminations parallel to the axis of the cylinder. The two split shells were then assembled to a shaft by machined dowel pins placed in the end plates having their axes perpendicular to the axis of the cylinder, and the end plates were then secured together by threaded fasteners. The principal disadvantages of this construction are the expense and time involved in machining the core and forming the integral end plate and core, and in the necessity of having to stock the separate end plates and integral end plates and cores from which the cylinders can be made. For very large sizes, cores have been fabricated from separate end plates welded to the ends of a tube forming a core. This also required machining to align the parts along the axis of the cylinder. Neither of these constructions permitted welding the laminations to the core except on the parting line of the core.
According to the present invention, first and second interchangeable end plates are used. These end plates are joined together by a plurality of core bars. Preferably, the core bars are rods having circular cross section; and their axes are parallel to and approximately equidistant from the axis of the cylinder. The core bars are located to engage the inner curved edge of the laminations when the laminations are assembled to the core. The spacing of the core bars permits greater access to the inner edges of the laminations; and the location of the core bars in engagement with the inner edges of the laminations provides a better means of securing the laminations to the core than was possible in prior commercial constructions. The core bars are welded to the inner edges of the laminations for holding the laminations firmly in place. Tie rods extending through the laminations are also secured to the end plates to hold the laminations. Both the tie rods and the core bars have their ends threaded, in a preferred embodiment, and are secured to the end plates by nuts which are counter sunk into the outer surfaces of the end plates.
The inner edges of the laminations, the core bars and the welds are all embedded in a layer of epoxy resin which serves to further strengthen the assembly and forms a part of the core. The split cylinders are secured together with shoulder bolts received in aligned, machined bores extending through abutting end plates. Thus, the shoulder bolts perform the dual function of aligning the end plates and securing the split cylinders together.
The present invention thus obviates the need of machining a special core from a large piece of steel stock or of fabricating a weldment to form the core, while providing a better means of securing the laminations. Further, for a given size printing cylinder, the end plates are interchangeable, thereby reducing the inventory of parts that must be stocked for manufacture.
Other features and advantages of the present invention will be apparent from the following description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.